Photophysical and Photocatalytic Properties of Core-Ring Structured

Jul 21, 2009 - The innovative core-ring structured NiCo2O4 nanoplatelets were found to be novel and promising ... View: PDF | PDF w/ Links | Full Text...
0 downloads 0 Views 5MB Size
Download PDF
J. Phys. Chem. C 2009, 113, 14083–14087

14083

Photophysical and Photocatalytic Properties of Core-Ring Structured NiCo2O4 Nanoplatelets Bai Cui,†,‡ Hong Lin,*,† Yi-zhu Liu,† Jian-bao Li,† Peng Sun,† Xiao-chong Zhao,† and Chuan-jie Liu† State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ReceiVed: January 2, 2009; ReVised Manuscript ReceiVed: May 30, 2009

The innovative core-ring structured NiCo2O4 nanoplatelets were found to be novel and promising photocatalysts. The physical and photophysical properties of the photocatalyst were characterized by SEM, TEM, XPS, UV-vis absorption, and photoluminescence, respectively. The core-ring NiCo2O4 nanoplatelets were composed of much smaller nanocrystallines, with an average size of 80-150 nm, compared to the ordinary NiCo2O4 prepared through a conventional hydroxide decomposition method. Moreover, the optical band gap energies of the core-ring NiCo2O4 nanoplatelets were estimated to be 2.06 and 3.63 eV from the UV-vis absorption spectra. The core-ring structured NiCo2O4 photocatalyst exhibited a much higher photocatalytic activity for the degradation of methylene blue than the ordinary NiCo2O4 and TiO2 under visible light irradiation (>420 nm). This enhanced photocatalytic activity of the core-ring NiCo2O4 nanoplatelets was attributed to their higher optical absorption ability, smaller particle size, and more active internal electron transitions. On the basis of all the results, the band structure of the photocatalyst was discussed. 1. Introduction NiCo2O4 materials show excellent electrocatalytic activities for many electrode reactions from O21–7 and Cl28 evolution to O2 reduction.9 They also appear to be an effective negative electrode of sodium-ion or lithium-ion battery,10,11 and show unusual ferromagnetic properties.12 NiCo2O4 is generally regarded to adopt a spinel structure in which nickel occupies the octahedral sites and cobalt is distributed over both octahedral and tetrahedral sites. In its structure, the solid state redox couples Co3+/Co2+ and Ni3+/Ni2+ are present, which provide a notable catalytic activity. Many researchers have focused on the study of the electrocatalytic activity of NiCo2O4.1–9 However, up to now, we still have little knowledge about the photocatalytic properties of this electroactive material. The visible-light responsive photocatalyst appears the ideal photocatalyst because visible light accounts for 43% of solar energy. However, there have been only a few studies of visiblelight catalysts. Some TiO2-based photocatalysts, such as reduced TiOx (TiO2-xNx) and Au-TiO2 photocatalyst,13,14 have been used in the degradation of methylene blue (MB, a model dye contaminant that evaluates the activity of a photocatalyst13,15) under visible light; however, the efficiency of TiO2-based photocatalysts is limited by the light absorption characteristics of TiO2. Recently, a series of oxide semiconductors with typical spinel structure, such as MIn2O4 (M ) Ca, Sr, Ba),16,17 were developed for MB degradation under visible light. The spinel oxides such as BaAl2O4,18 ZnFe2O4,19 CuMn2O4, and ZnMn2O420 are also discovered to be attractive photocatalysts for splitting water to generate hydrogen. By analogy, we think that NiCo2O4 may have some photocatalytic activity. Nanostructured materials are rarely reported in NiCo2O4, and fewer studies are carried out on the influence of nanostructure * To whom correspondence should be addressed. Phone/Fax: (+86)1062772672. E-mail: [email protected]. † Tsinghua University. ‡ Imperial College London.

on the catalytic activity. In previous studies, the spinel NiCo2O4 nanofibers with diameters of 50-100 nm were prepared by using sol-gel processing and electrospinning technique.21 Yang et al.22 described a preparation of a porous nanosheet-stacked NiCo2O4 composite electrode using a novel electrophoretic deposition (EPD) calcination method; the electrode is composed of regular hexagonal nanosheets with a diameter of about 200 nm and the thickness of several tens of nanometers. Very recently, we have found the innovative core-ring nanostructured NiCo2O4 nanoplatelets1 synthesized by the coprecipitation decomposition method. It is found that the yield of these core-ring structured NiCo2O4 nanoplatelets could be higher than 80% by controlling the experimental conditions. It is revealed by energy dispersive spectroscopy (EDS) analysis that the cation concentration of Co (Ni) in this structure gradually increases (decreases) from the ring to the core. The core-ring nanostructure provides an increased active surface area and provides a large number of active surface Co atoms, which are probably the main reasons for the excellent electrocatalytic activity. In this study, the as-synthesized core-ring structured NiCo2O4 nanoplatelets were characterized by a scanning electronic microscope (SEM), a transmission electronic microscopy (TEM), X-ray photoelectron spectroscopy (XPS), optical UV-vis absorption measurements, and photoluminescence measurements. The photocatalytic degradation of methylene blue (MB) dye under UV and visible light was investigated to evaluate the photocatalytic activity of the core-ring structured NiCo2O4 nanoplatelets. To understand their photocatalytic properties, an electronic band structure of NiCo2O4 is proposed. A description of the correlation between the photocatalytic activity and the electronic band structure is attempted. 2. Experimental Section The core-ring structured NiCo2O4 nanoplatelets were synthesized via hydroxide decomposition method, as we have described elsewhere.1 In brief, Ni(NO3)2 · 6H2O and Co(NO3)2 ·

10.1021/jp900028t CCC: $40.75  2009 American Chemical Society Published on Web 07/21/2009

14084

J. Phys. Chem. C, Vol. 113, No. 32, 2009

6H2O were dissolved in deionized water in the molar proportion of 1:4. The resulting solution was stirred under flowing N2 atmosphere at room temperature for 1 h, followed by the rapid addition of NaOH (2 M, 25.0 mL) in 5 min, during which a large amount of Ni-Co hydroxide coprecipitate was formed while the remaining filtrate had a pH of 14.1. To prevent the crystalline growth, this coprecipitate was immediately filtered off, washed with deionized water and alcohol, and dried at room temperature under vacuum overnight. The coprecipitates were heated to 200 °C for 1 h in air at a rate of 6 deg min-1 to prepare the core-ring structured NiCo2O4 nanoplatelets. For comparison, ordinary NiCo2O4 (with the stoichiometric ratio of Ni:Co )1: 2) powders were also prepared by a conventional hydroxide decomposition method. In a typical preparation for ordinary NiCo2O4, Ni(NO3)2 · 6H2O and Co(NO3)2 · 6H2O were dissolved in deionized water in the stoichiometric molar proportion of 1:2 (0.3 M, 50 mL), followed by the addition of NaOH (2 M, 25.0 mL). The resulting hydroxide coprecipitate was filtered and washed with deionized water and alcohol, dried at room temperature under vacuum overnight, and then calcined at 300 °C for 3 h in air. The morphologies and microstructures were investigated by using a scanning electron microscope (SEM) (JSM-6301, JEOL, Japan) and transmission electron microscopy (TEM) (JEM 2010, JEOL, Japan). The X-ray photoelectron spectrum (XPS) was collected with a X-ray photoelectron spectrophotometer (XPS, AEM PHI5300, PE). UV-visible absorption spectra were recorded with a spectrophotometer (UNICAM, UV 500) at room temperature. The solutions in alcohol were prepared ultrasonically for the UV-visible measurements. The photoluminescence (PL) spectra of the photocatalyst were detected with a Cary Eclipe fluorescence spectrometer (Varian, USA) with use of a Xe lamp as the excitation source. The photocatalytic activities of core-ring structured NiCo2O4 nanoplatelets, ordinary NiCo2O4, and TiO2 (P25, Degussa, Germany) were measured by the degradation of methylene blue (MB) in an aqueous solution. A black light lamp with a peak of 351 nm and a light power of 5.4 mW cm-2 was used as a UV source (320 nm < λ < 400 nm); a 300 W halogen lamp (CMH-250) with a filter whose cutoff wavelength is 420 nm was used as a visible-light source (λ > 420 nm). A 0.1 g sample of photocatalyst was suspended in a 50 mL aqueous solution of 2.5 × 10-5 M MB. Before the photocatalytic experiment, the suspension was stirred in the dark for 1 h. The concentration of the MB solution was monitored every 1 h by measuring the maximum absorbance of MB at 664 nm, using the UV-vis spectrum.23 3. Results and Discussion 3.1. Characterization of Materials. Figure 1 shows the SEM image of the as-synthesized core-ring structured NiCo2O4 nanoplatelets. Figure 2a shows the TEM image of these corering NiCo2O4 nanoplatelets. More than 80% hexagonal nanoplatelets have rings. Figure 2b shows a magnified view of a core-ring NiCo2O4 nanoplatelet. The observed core-ring NiCo2O4 nanoplatelet is hexagonal, small-sized, and ranges from 80 to 150 nm. The ring is 10-20 nm in width and the core is 60-90 nm in diameter. The XPS spectra of the core-ring structured NiCo2O4 nanoplatelets are shown in Figure 3a-c. By using a Gaussian fitting method, the O 1s emission spectrum can be divided into two main peaks (Figure 3a). The component at 529.5 eV is typical of metal-oxygen bonds, while the component at 531.4 eV is associated with oxygen ions in low coordination at the surface.24

Cui et al.

Figure 1. SEM image of the as-synthesized core-ring structured NiCo2O4 nanoplatelets.

Figure 2. (a) TEM image of the core-ring structured NiCo2O4 nanoplatelets. (b) A magnified view of a core-ring structured NiCo2O4 nanoplatelet.

The Co 2p spectrum (Figure 3b) was best fitted considering two spin-orbit doublets characteristic of Co2+ and Co3+ and two shakeup satellites (identified as “Sat.”). The Ni 2p spectrum (Figure 3c) was best fitted considering two spin-orbit doublets characteristic of Ni2+ and Ni3+ and two shakeup satellites. These results show that the surface of the core-ring structured NiCo2O4

Core-Ring Structured NiCo2O4 Nanoplatelets

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14085

(Rhν)n ) K(hν - Eg)

Figure 3. (a) XPS spectrum of the O 1s core levels for the corering NiCo2O4. (b) XPS spectrum of the Co 2p core levels for the core-ring NiCo2O4. (c) XPS spectrum of the Ni 2p core levels for the core-ring NiCo2O4.

Figure 4. UV-vis absorption spectra for the core-ring NiCo2O4 and NiCo2O4 (ordinary).

nanoplatelet has a composition containing Co2+, Co3+, Ni2+, and Ni3+, which is in good agreement with the results in the literature for NiCo2O4.24–26 The darkish core-ring structured NiCo2O4 nanoplatelets show good absorbance of light between the wavelengths of 300 and 800 nm, as shown in Figure 4. It could be seen that between the wavelengths of 300 and 560 nm, the core-ring NiCo2O4 show noticeably higher absorption of light than the ordinary NiCo2O4. Between the wavelengths of 560 and 800 nm the core-ring NiCo2O4 show slightly lower absorption of light than the ordinary NiCo2O4. The absorption band gap energy, Eg, can be determined by the following equation:27,28

(1)

where hν is the photoenergy, R is the absorption coefficient, K is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The (Rhν)2-hν curves for the core-ring NiCo2O4 nanoplatelets and the ordinary NiCo2O4 are shown in Figure 5. The band gap energies of the core-ring NiCo2O4 nanoplatelets are calculated to be 2.06 and 3.63 eV, by the extrapolation of eq 1. The band gap energies of the ordinary NiCo2O4 are calculated similarly to be 1.97 and 3.40 eV. No linear relation was found for n ) 1/2, suggesting that NiCo2O4 are semiconducting with direct transitions at these energy levels. It is well-known that the Eg of a semiconductor increases with a decrease in grain size.29,30 In the current study, the NiCo2O4 nanoplatelets were composed of much smaller nanocrystallines, with an average size of 80-150 nm, compared to the ordinary NiCo2O4 (the average size is from 150 to 200 nm, which is summarized from sufficient SEM and TEM observations). Therefore, the core-ring NiCo2O4 has higher band gap energies than the ordinary NiCo2O4. 3.2. Photocatalytic Activity of the Prepared Catalysts. The photocatalytic activities of core-ring NiCo2O4 nanoplatelets, the ordinary NiCo2O4, and TiO2 (P25) were compared by measuring the degradation of MB aqueous solution. Under the visible light (Figure 6a), the order of photocatalytic activity is as follows: core-ring NiCo2O4 > ordinary NiCo2O4 > TiO2. The photocatalytic activity of the core-ring NiCo2O4 is better than that of the ordinary NiCo2O4, and is much better than that of TiO2. MB degradation over TiO2 under visible light is based on the dyesensitized process.13,31 The dye-sensitized process over NiCo2O4 assisted by adsorbed MB dye under visible light irradiation also might be possible. However, the dye-sensitized process requires that the lowest unoccupied molecular orbital (LUMO) of MB can couple well with the conduction band of the NiCo2O4 photocatalyst.32 Since such coupling is not quite clear and is difficult, it can be assumed that the direct photocatalytic decomposition over the NiCo2O4 photocatalyst is the main process for the MB degradation. Under the UV light (Figure 6b), the order of photocatalytic activity is as follows: TiO2 > core-ring NiCo2O4 > ordinary NiCo2O4. However, under short-time irradiation, the catalytic activity of core-ring NiCo2O4 is higher than that of TiO2. When irradiated for less than 20 min, the order of photocatalytic activity is core-ring NiCo2O4 ≈ ordinary NiCo2O4 > TiO2; when irradiated for 40 min, the order is core-ring NiCo2O4 > TiO2 > ordinary NiCo2O4. It can be concluded that under the UV light the core-ring NiCo2O4 also show higher photocatalytic activity than the ordinary NiCo2O4, and under short-time UV light irradiation its activity is even better than that of TiO2. 3.3. Discussion of the Band Structure. It is generally known that in the spinel structure of NiCo2O4, the divalent cations Co2+ (half of Co cations) occupy the tetrahedral sites, whereas the trivalent Ni3+ cations and the trivalent Co3+ cations (the other half of Co cations) reside in the octahedral sites, though the detailed description of the distribution of the cationic sites is still a matter of some uncertainty.12,33 The general description of the electron configuration in this structure is tetrahedral high spin Co2+ (eg4t2g3), octahedral low spin Co3+ (t2g6), and Ni3+ (t2g6eg1). The band structure of NiCo2O4 can be defined by taking the O 2p obital as the valence band and the Ni 3d and Co 3d orbitals as the conduction band. Since the high level Co 3d-eg orbital is partially filled, the excitation of electrons from the Co 3d-t2g orbital to the Co 3d-eg orbital is possible. Similarly, the excitation of electrons from the Ni 3d-t2g orbital to the empty

14086

J. Phys. Chem. C, Vol. 113, No. 32, 2009

Figure 5. (Rhν) -hν curve for the core-ring NiCo2O4 and NiCo2O4 (ordinary). 2

Cui et al.

Figure 7. Schematic illustration of the electronic band structure of NiCo2O4. Different photoexcitations of electrons: (1) from O 2p to Co 3d-eg (or Ni 3d-eg), (2) from O 2p to Co 3d-t2g (or Ni 3d-t2g), and (3) from Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg) are induced by the irradiation of UV and visible lights accordingly. (See the text for details.)

Figure 8. PL emission spectra for the core-ring NiCo2O4 and the ordinary NiCo2O4 photocatalysts excited at 254 nm.

Figure 6. (a) Photocatalytic MB degradation under visible light (λ > 420 nm) at room temperature over the core-ring NiCo2O4, NiCo2O4 (ordinary), and TiO2 (P25). (b) Photocatalytic MB degradation under UV light (320 nm < λ < 400 nm) at room temperature over the corering NiCo2O4, NiCo2O4 (ordinary), and TiO2 (P25).

Ni 3d-eg orbital is possible. The internal electron transitions in Co 3d and Ni 3d orbitals must make a significant contribution to the band structure and thus the photocatalytic activity of NiCo2O4, as in the case of Ni-doped InTaO4 compounds34 and BaCr2O4.18 Taking into account the band gap energies estimated from the UV-vis absorption spectrum, a simplified electronic band structure of NiCo2O4 was proposed and schematically illustrated in Figure 7. It shows that three kinds of photoexcitation of electrons are possible with respect to the energy of incident photons: (1) from O 2p to Co 3d-eg (or Ni 3d-eg), (2) from O 2p to Co 3d-t2 g (or Ni 3d-t2g), and (3) from Co 3d-t2g to Co 3d-eg (or from Ni 3d-t2g to Ni 3d-eg). The photoexcitation of electrons (1) and (2) corresponds to two direct energy band gaps of NiCo2O4, respectively. The two direct energy band gaps of the ordinary NiCo2O4 are 3.40 and 1.97 eV, while the two direct energy band gaps of the core-ring NiCo2O4 are 3.63 and 2.06 eV. The photoluminescence (PL) spectra of the semiconductor are useful to disclose the migration, transfer, and recombination processes of the photogenerated electron-hole pairs in the

semiconductor because PL emission results from the recombination of free carriers.35,36 The PL spectra of the core-ring NiCo2O4 and the ordinary NiCo2O4 photocatalysts are presented in Figure 8. In the PL spectrum of the core-ring NiCo2O4, three main emission peaks appear at about 361, 615, and 738 nm, which correspond to the emission photon energies of 3.43, 2.02, and 1.68 eV, respectively. The three emission peaks are ascribed to the recombination of the hole formed in O 2p and the electron in the Co 3d-eg (or Ni 3d-eg), the recombination of the hole formed in O 2p and the electron in Co 3d t2g (or Ni 3d-t2g), and the recombination of the hole formed in Co 3d t2g (or Ni 3d-t2g) and the electron in Co 3d-eg (or Ni 3d-eg), respectively. Thus, the first two emission peaks (at 361 and 615 nm) can be ascribed to the emission of band transitions of two direct energy band gaps. The difference between the band gap energies (3.63 and 2.06 eV) and the emission peak energies (3.43 and 2.02 eV) is caused by the Stokes shift due to the Franck-Condon effect.37,38 In the PL spectrum of the ordinary NiCo2O4, two peaks appear at 392 and 598 nm, which correspond to the recombination of the hole formed in O 2p and the electron in the Co 3d-eg (or Ni 3d-eg), and the recombination of the hole formed in O 2p and the electron in Co 3d t2g (or Ni 3d-t2g). Compared with the PL spectrum of the core-ring NiCo2O4, the emission peak corresponding to the recombination of the hole formed in Co 3d t2g (or Ni 3d-t2g) and the electron in Co 3d-eg (or Ni 3d-eg) could not be observed in the PL spectrum of the ordinary NiCo2O4. This means that the internal electron transitions (from Co 3d-t2g to Co 3d-eg and from Ni 3d-t2g to Ni 3d-eg) in the core-ring NiCo2O4 are more active than those in the ordinary NiCo2O4. The internal electron transitions are found to play an important role in the photoexcitation and photocatalytic activity, as in the case of Ni-doped InTaO4 compounds34 and BaCr2O4.18

Core-Ring Structured NiCo2O4 Nanoplatelets Photocatalysis with oxide semiconductor involves the absorption of band gap photons, the separation of electron-hole pairs, and the redox reaction on the surface of catalyst. The excitation of an electron from valence band to conduction band is initiated by the absorption of a photon with energy equal to or greater than the band gap energy of the semiconductor. Under UV light irradiation, for the core-ring NiCo2O4, an electron generated by a photon with the energy >3.63 eV (λ < 342 nm, according to the following equation: λ (nm) ) 1240/Eg (eV)) can be directly excited from the O 2p valence band to the Co 3d-eg (or Ni 3d-eg) conduction band. Under visible light irradiation, an electron generated by a photon with the energy >2.06 eV (342 nm < λ < 601 nm) can be directly excited from the O 2p valence band to the Co 3d-t2g (or Ni 3d-t2g) conduction band. In addition, an electron can be first excited from the O 2p valence band to the Co 3d-t2g (or Ni 3d-t2g) conduction band, and then transferred from the Co 3d-t2g (or Ni 3d-t2g) conduction band to the Co 3d-eg (or Ni 3d-eg) conduction band. Eventually, the photogenrated electrons and holes could participate in the complex redox reaction on the surface of the catalyst, leading to the photocatalytic degradation of MB.39–41 Compared with the ordinary NiCo2O4, the core-ring NiCo2O4 nanoplatelets exhibit a much smaller particle size, higher optical absorption ability, and more active internal electron transition in 3d orbitals. For nanoparticles, the diffusion length of electrons and holes (e-/h+) from the bulk to the surface is short, which helps to accelerate the migration rate of e-/h+ to the surface of the nanoparticle to participate in the reaction process.42,43 The higher optical absorption ability is effective in generating charge carriers. In addition, the internal electron transitions are found to play an important role in the photoexcitation and photocatalytic activity. Therefore, the higher MB degradation efficiency of the core-ring NiCo2O4 nanoplatelets is attributed to its smaller particle size, its greater ability to absorb visible light, and more active internal electron transitions compared to the ordinary NiCo2O4 sample. 4. Conclusions The core-ring NiCo2O4 nanoplatelets were characterized and their photocatalytic properties were investigated. The core-ring NiCo2O4 nanoplatelets were composed of much smaller nanocrystallines with an average size of 80-150 nm compared to the ordinary NiCo2O4 prepared through a conventional hydroxide decomposition method. From the UV-vis absorption spectra, the optical band gap energies of the core-ring NiCo2O4 nanoplatelets were 2.06 and 3.63 eV. The core-ring NiCo2O4 nanoplatelets exhibited much higher photocatalytic activity for the degradation of MB dye solution under visible light irradiation (>420 nm). This enhanced photocatalytic activity of the corering NiCo2O4 nanoplatelets was attributed to their higher optical absorption ability, smaller particle size, and more active internal electron transitions. On the basis of these experimental results, a possible band structure was suggested, where the valence band of the photocatalyst was the O 2p level and conduction bands were composed of the Co 3d-eg (and Ni 3d-eg) level and the Co 3d-t2g (and Ni 3d-t2g) level, respectively. Acknowledgment. The authors would like to express their gratitude for the support of the Ministry of Science & Technology of China (863 Program, 2006AA03Z218; 973 Program, 2007CB607504) and the National Natural Science Foundation of China (NSFC, 50572051, 50672041).

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14087 References and Notes (1) Cui, B.; Lin, H.; Li, J. B.; Li, X.; Yang, J.; Tao, J. AdV. Funct. Mater. 2008, 18, 1440. (2) Rasiyah, P.; Tseung, A. C. C. J. Electrochem. Soc. 1983, 130, 2384. (3) Singh, R. N.; Koenig, J. F.; Poillerat, G.; Chartier, P. J. Electrochem. Soc. 1990, 137, 1408. (4) Bocca, C.; Barbucci, A.; Delucchi, M.; Cerisola, G. Int. J. Hydrogen Energy 1999, 24, 21. (5) Suffredini, H. B.; Cerne, J. L.; Crnkovic, F. C.; Machado, S. A. S.; Avaca, L. A. Int. J. Hydrogen Energy 2000, 25, 415. (6) Marsan, B.; Fradette, N.; Beaudoin, G. J. Electrochem. Soc. 1992, 139, 1889. (7) Chi, B.; Li, J. B.; Han, Y. S.; Chen, Y. J. Int. J. Hydrogen Energy 2004, 29, 605. (8) Bogglo, R.; Carugati, A.; Lodi, G.; Trasatti, S. J. Appl. Electrochem. 1985, 15, 335. (9) Singh, R. N.; Koenig, J. F.; Poillerat, G.; Chartier, P. J. Electroanal. Chem. 1991, 314, 241. (10) Alcantara, R.; Jaraba, M.; Lavela, P.; Tarado, J. L. Chem. Mater. 2002, 14, 2847. (11) Chardvick, A. V.; Savin, S. L. P.; Fiddy, S.; Alcantara, R.; Lisbona, D. F.; Lavela, P.; Ortiz, G. F.; Tirado, J. L. J. Phys. Chem. C 2007, 111, 4636. (12) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rios, E. I.; Palmer, H. M.; Greaves, C.; Berry, F. J. J. Mater. Chem. 2001, 11, 3087. (13) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (14) Li, X.; Li, F. J. Phys. Chem. B 1999, 103, 4862. (15) Belhekar, A. A.; Awate, S. V.; Anand, R. Catal. Commun. 2002, 3, 453. (16) Tang, J.; Zou, Z.; Ye, J. Chem. Mater. 2004, 16, 1644. (17) Tang, J.; Zou, Z.; Yin, J.; Ye, J. Chem. Phys. Lett. 2003, 382, 175. (18) Wang, D.; Zou, Z.; Ye, J. Chem. Phys. Lett. 2003, 373, 191. (19) Valenzuela, M. A.; Bosch, P.; Jimenez-Becerrill, J.; Quiroz, O.; Paez, A. I. J. Photochem. Photobiol. A 2002, 148, 177. (20) Bessekhouad, Y.; Trari, M. Int. J. Hydrogen Energy 2002, 27, 357. (21) Guan, H. Y.; Shao, C. L.; Liu, Y. C.; Yu, N.; Yang, X. H. Solid State Commun. 2004, 131, 107. (22) Yang, J.; Li, J. B.; Lin, H.; Yang, X. Z.; Tong, X. G.; Guo, G. F. J. Appl. Electrochem. 2006, 36, 94. (23) Chi, B.; Zhao, L.; Jin, T. J. Phys. Chem. C 2007, 111, 6189. (24) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rı´os, E.; Berry, F. J. J. Solid State Chem. 2000, 153, 74. (25) Kim, J. G.; Pugmire, D. L.; Battaglia, D.; Langell, M. A. Appl. Surf. Sci. 2000, 165, 70. (26) Thissen, A.; Ensling, D.; Ferna´ndez Madrigal, F. J.; Jaegermann, W.; Alca´ntara, R.; Lavela, P.; Tirado, J. L. Chem. Mater. 2005, 17, 5202. (27) Kubelka, P.; Munk, F. Z. Tech. Phys. (Leipzig) 1931, 12, 593. (28) Wang, X.; Chen, X. Y.; Gao, L. S.; Zheng, H. G.; Zhang, Z. D.; Qian, Y. T. J. Phys. Chem. B 2004, 108, 1640. (29) Satoh, N.; Nakashima, T.; Kamikura, K.; Yamamoto, K. Nat. Nanotechnol. 2008, 3, 106. (30) Gu, F.; Li, C. Z.; Hu, Y. J.; Zhang, L. J. Cryst. Growth 2007, 304, 369. (31) Xue, M.; Huang, L.; Wang, J.; Wang, Y.; Gao, L.; Zhu, J.; Zou, Z. Nanotechnology 2008, 19, 185604. (32) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (33) Lenglet, M.; Guillamet, R.; Durr, J.; Gryffroy, D.; Vandenberghe, R. E. Solid State Commun. 1990, 74, 1035. (34) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (35) Long, M. C.; Cai, W. M.; Cai, J.; Zhou, B. X.; Chai, X. Y.; Wu, Y. H. J. Phys. Chem. B 2006, 110, 20211. (36) Tang, J.; Zou, Z.; Ye, J. J. Phys. Chem. B 2003, 107, 14265. (37) Serpone, N.; Lawless, D.; Khairutdinovt, R. J. Phys. Chem. 1995, 99, 16646. (38) Liu, B.; Zhao, X.; Zhang, N.; Zhao, Q.; He, X.; Feng, J. Surf. Sci. 2005, 595, 203. (39) Kuo, W. S.; Ho, P. H. Chemosphere 2001, 45, 77. (40) Mohapatraa, P.; Parida, K. M. J. Alloys Compd. 2006, 258, 118. (41) Shimizua, N.; Oginob, C.; Dadjourc, M. F.; Murata, T. Ultrason. Sonochem. 2007, 14, 184. (42) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (43) Cho, I.; Lee, S.; Noh, J.; Choi, G.; Jung, H.; Kim, D.; Hong, K. J. Phys. Chem. C 2008, 112, 18393–18398.

JP900028T